In gas turbine engines, barriers or walls, usually called duct liners, are installed between the hot exhaust gas flow and surrounding engine material and components. To conduct heat effectively and avoid unwanted additions to engine size and weight, these liners are fabricated from thin metal sheets. Physical characteristics of these liners, mainly shape, can inhibit their capacity to conduct heat away from local liner hot spots, which can develop under certain conditions. These liners are exposed to extremely high temperatures, and this creates unusual expansion responses, among them warping and buckling. Those changes can produce hot spots if they restrict cooling air flow through the air metering passages that are often used in current liners.
U.S. Pat. No. 4,887,663, which is assigned to the assignee of this application, and U.S. Pat. No. 4,800,718 illustrate conventional schemes for constructing improved liners for gas turbine engine exhausts. The liner discussed in U.S. Pat. No. 4,800,718 is a complex design of the type known to employ "louvers" in air ducts in conjunction with air dams. The air duct includes an up-steam duct wall that terminates in a downstream edge or lip. A second duct wall is spaced radially outward relative to the first surface lip and defines an elongated louver nozzle through which the cooling air that enters the supply orifices (metering holes) exits. Among the shortcomings of this designs philosophy is that the liner can be very expensive to fabricate and repair, owing to the complex design and the number of components. Heat resistant coatings, used in many applications in gas turbine engines for their beneficial thermal and rear resistance, cannot be applied to liners with that a design, at least not without seriously risking closing off the downstream lip with coating material, which would restrict cooling air flow through the liner. Reducing the cost and complexity of these liners presents obvious benefits, but being able to coat liners without diminishing cooling efficiency and increasing liner weight offers significant improvement. One way to apply coatings is by "plasma spray." This done in coating some exhaust nozzle parts, for instance, the aft divergent flap. One type of coating particularly suited for this environment is Spec PWA 265 coating by United Technologies Corporation, a two-layer, plasma sprayed coating consisting of a nickel bond layer and a yttrium oxide stabilized zirconium oxide ceramic layer. Coatings increase liner operating life by protecting the liner structure from direct contact with hot/corrosive exhaust gases. Coating also simplifies liner repair. A thermally worn-out or sacrificial liner coating simply may be reapplied instead of replacing the entire liner, the conventional approach at this time.